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As a senior process engineer, share some understanding of the injection molding process

The mold temperature affects the molding cycle and forming quality, and in actual operation, the minimum appropriate mold temperature of the material used is set from the beginning, and then adjusted appropriately according to the quality condition.

To be correct, mold temperature refers to the temperature of the mold cavity surface when the molding is carried out, and it is important to not only maintain the appropriate temperature but also distribute it evenly in the mold design and forming engineering conditions.

Uneven mold temperature distribution leads to uneven shrinkage and internal stress, which makes the molding port prone to deformation and warpage.

Increasing the mold temperature can achieve the following effects:

Add the crystallinity of the molded product and a relatively uniform structure. 

The molding shrinkage is more sufficient, and the post-shrinkage is reduced.

Improve the strength and heat resistance of molded products.

Reduce internal stress residue, molecular alignment and deformation.

Reduce the flow reactance during filling and reduce pressure loss

Make the appearance of the molded product more shiny and good 

Increase the chance of burrs on molded products. 

Increase near gate sites and reduce the chance of dents in far gate sites.

Reduce the degree of pronouncement of the binding line

Increase the cooldown time 

Pressure regulation during injection moldingWhether it is an oil pressure or an electric injection molding machine, all movements during the injection molding process generate pressure. Proper control of the required pressure can produce a finished product of reasonable quality.

Pressure control and metering system On hydraulic injection molding machines, all movements are performed by the oil circuit responsible for: 

The screw rotates in the plasticizing stage.

Slide forehearth (nozzle close to nozzle bushing)

Axial movement of the injection screw during injection and packing 

The substrate is closed on the injection bar until the toggle rod is fully extended or the piston clamping stroke is complete.

Initiates the assembly ejector bar to eject the part

On a full voltage machine, all movements are performed by a brushless synchronous motor equipped with a permanent magnet. Ball bearing screws, which have always been used in the machine tool industry, transform rotary motion into linear motion. The efficiency of the entire process depends partly on the plasticizing process, in which the screw plays a crucial role.

The screw must ensure that the material is melted and homogenized. This process can be adjusted with the help of backpressure to avoid overheating. The mixing elements must not produce excessively high flow rates, which would lead to polymer degradation.

Each polymer has a different maximum flow rate, and if this limit is exceeded, the molecules stretch and the polymer backbone breaks. However, the focus is still on controlling the forward axial movement of the screw during injection and packing. The subsequent cooling process, including internal stress, tolerances and warpage, is important to ensure product quality. This is all determined by the quality of the mold, especially when it comes to optimizing the cooling forehearth and ensuring effective closed-loop temperature regulation. The system is completely self-contained and does not interfere with mechanical adjustment.

Mold movements such as mold closure and ejection must be precise and efficient. Velocity distribution curves are often used to ensure that moving parts are accurately approached. Contact retention is adjustable. Therefore, it can be concluded that the quality of the product is mainly determined by the system that controls the forward movement phase of the screw, without considering energy consumption and mechanical reliability, and the additional conditions are the same (such as mold quality). On hydraulic injection molding machines, this adjustment is achieved by detecting the oil pressure.

Specifically, the oil pressure is activated by the control panel and a set of valves is activated, and the fluid is activated by the manipulator, and is regulated and released.  

Injection speed control includes options such as open-loop control, semi-closed-loop control, and closed-loop control. Open loop systems rely on shared proportional valves. The proportional tension is applied to the required proportion of the fluid, so that the fluid creates pressure in the injection barrel and allows the injection screw to move at a certain forward speed.

The semi-closed-loop system adopts a closed-loop proportional valve. The loop closes where the closing port is located, which controls the proportion of oil flow through movement within the valve. The closed-loop system closes at the screw translation speed. Speed sensors (usually potentiometer types) are used in closed-loop systems to periodically detect tension drops. The oil flowing out of the proportional valve is adjusted to compensate for the resulting speed deviation.

Closed-loop control relies on dedicated electronics integrated into the machine. Closed-loop pressure control ensures uniform pressure during the injection and packing phases, as well as uniform backpressure throughout cycles. The proportional valve is regulated by the detected pressure value, and deviations are compensated according to the set pressure value.

In general, the hydraulic pressure can be monitored, but detecting the melt pressure in the nozzle or mold cavity is another effective method. A more reliable solution is to manage proportional valves by reading nozzle or cavity pressure readings. The addition of temperature detection to pressure detection is particularly beneficial for process management.

Knowing the actual pressure that the material can withstand also helps predict the actual weight and dimensions of the molded part based on set pressure and temperature conditions. In fact, by changing the holding pressure value, more material can be introduced into the cavity to reduce part shrinkage and meet design tolerances, including preset injection shrinkage. As the melting conditions approach, semicrystalline polymers show a great change in specific volume. For this, overcharging does not prevent the ejection of the part.

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